1. Field of the Invention
The present invention relates to embedded capacitors. More particularly, the present invention provides a structure and method for embedding capacitors in multi-chip modules.
2. Description of the Prior Art
A bypass capacitor safeguards a power system from signal induced fluctuation by supplying a reservoir of charge on multi-chip modules (MCMs) in close proximity to semiconductor chips. Currently high performance mainframe computers utilize hundreds of surface mounted LICAs (Low Inductance Capacitor Arrays) on MCMs in close proximity to semiconductor chips. These LICAs consume valuable area on MCMs and currently cost from approximately $3 to about $10 each.
As MCMs frequencies increase, parasitic series inductances in the LICAs increase and the utility of LICAs as bypass capacitors decreases. When the series inductance increases, the capacitors ability to rapidly supply charge to a semiconductor chip over a given distance diminishes. Furthermore, the distance from switching drivers at the center of a semiconductor chip is sufficiently far from the LICA capacitors that the flight time prevents these capacitors from attenuating the early switching noise at the high frequencies. As frequencies increase to ˜1 GHz the impedence of the parasitic inductance increases to the point that it dominates the LICA component performance. Thus, LICAs may have limited utility beyond a frequency of about 1 GHz.
Recognizing that the inductances and distances between the LICAs and a chip are greater than the inductances and thickness of thin film MCMs, a solution to the bypass capacitor problem at GHz frequencies is to embed thin film capacitors within thin film MCMs. Such a solution can provide high frequency (>1 GHz) performance and open up more area for additional active components on MCMs.
When using a buildup process to make an MCM, some defects in embedded thin film capacitor layers are likely to occur. Such defects could become power to ground shorts that render the MCM useless. In order to avoid such an occurrence, one would like to be able to test all capacitors prior to connecting them up to the subsequent build up layers. This is very difficult in build-up structures without having a separate mask available for every conceivable combination of good and defective capacitors. For a large number of capacitors, such a multi-mask approach becomes very impractical. Therefore, what is needed and what has been invented is a structure and method which overcome the foregoing difficulties.